In a microdisplay-based projection display system, light from a light source may be modulated by the microdisplay as the light reflects off the surface of the microdisplay or passes through the microdisplay. Examples of commonly used microdisplays may include digital micromirror devices (DMD), deformable micromirror devices, transmissive or reflective liquid crystal, liquid crystal on silicon, ferroelectric liquid crystal on silicon, and so forth. In a DMD-based projection system, where large numbers of positional micromirrors may change state (position) depending on an image being displayed, light from the light source may be reflected onto or away from a display plane.
In operation, one type of projection display system using a DMD sequentially flooded the surface of the microdisplay with different colors of light from a high etendue light source, such as a lamp. The individual mirrors of the DMD were controlled to selectively turn each mirror (e.g., each pixel) either on or off as each of the different colors of light were shone on the DMD. When performed fast enough, the human eye integrates the sequential colors to form different colors and images.
Recent attempts have been made to use a scanning line in combination with a low etendue light source such as a laser. In these attempts, lines of colored light are scanned across the surface of a DMD. The individual mirrors were controlled to selectively turn each mirror on or off as the line of light passes over each respective mirror. This method, however, generally requires a very uniform intensity level over the length of the DMD in order to obtain a high quality image. If the intensity level is not uniform, then color streaking may occur in the displayed image. In order to obtain a line having a uniform intensity level, a long light pipe with complicated optics with strict alignment tolerances was required, thereby increasing costs and, possibly, decreasing yields during manufacturing.
Furthermore, the timing of the scan lines with respect to the DMD must be synchronized. As discussed above, the individual mirrors of the DMD are individually controlled to turn either on or off as the line pass over the DMD. Because the width of the line is small and the lines scan over the DMD many times a second, the timing and the position of the scan line is important in order to be able to correctly display an image. If the scan line is out of synch with the DMD, then either too much or not enough light may be redirected to the display image.
Therefore, there is a need for an illumination technique that provides a line of light having a substantially uniform intensity level for a light source in projection display systems.